Phase-locked loop (PLL) systems are used in various applications to match the phase and frequency of one signal to those of a reference signal. Although different PLLs may include different components, most PLLs include a phase-frequency detector (PFD), a low-pass filter, and an oscillator (e.g., a voltage-controlled oscillator (VCO)). Some of these components may introduce phase noise to the tuning signal that is provided to the VCO—for instance, distortions in the rising or falling edges of each pulse in the tuning signal. The VCO applies a gain to the tuning signal when producing the output signal, and this gain is undesirably applied to the phase noise as well.
To mitigate the amplification of phase noise, it may be desirable to reduce the VCO gain. Decreasing the VCO gain requires widening the voltage swing of the tuning signal provided to the VCO (e.g., a rail-to-rail swing). The PLL, however, may contain components—such as charge pumps and VCO varactors—that do not respond favorably to wide-swing tuning signals because they do not operate linearly at all tuning signal voltage values.